The present invention relates to cellular telephone systems and to determination of the RF signal propagation for optimization of the wireless system. More specifically, the present invention relates to detailed RF propagation modeling based upon measured RF propagation data.
The service area of a wireless communications system is partitioned into connected service domains known as cells, where radio telephone (cellular) users communicate, via radio links, with the base station serving the cell. The cells can be further partitioned into segments, typically three to a cell. The cell includes an antenna mast. Typically, if the cell has three sectors, the mast will have three sides, each facing a 120° segment of the perimeter of the mast. Typically, a number of directional antennae are mounted on each face of the mast to serve each segment of the cell site. The base station is coupled to the land line network.
Presently available commercial mobile communication systems typically include a plurality of fixed cells each of which transmits signals to and receives signals from mobile units within its communication area. In AMPS or FDMA systems, each base station is assigned a plurality of channels (each 30 KHz wide) within a frequency spectrum over which it can communicate with mobile units. A mobile unit within range of the base station communicates with the base station using these channels. Typically, the channels used by a base station are separated from one another in some manner (typically skipping 1, 7 or 21 intermediate channels) so that signals on any channel do not interfere with signals on another channel used by that base station. To accomplish this, an operator typically allots to a base station a group of channels each of which is widely separated from the next. The present invention will also operate with GSM and iDEN systems which do not rely on the same frequency divisions multiple access method.
In a common type of mobile system called Time Division Multiple Access (TDMA), which includes IS-54 and IS-136, GSM and iDEN each frequency channel is further time divided into additional channels within each frequency. Each base station sends and receives in bursts during some number of different intervals or time slots. These time intervals within frequency bands then effectively constitute the individual channels. In order to distinguish the channel divisions within a frequency and to distinguish channels of a common frequency between overlapping cells digital codes are used. For example, IS-136 utilizes Digital Verification Color Codes unique to a channel at a cell. GSM uses Base Station identification codes.
In order to allow mobile units to transmit and receive telephone communications as the units travel over a wide geographic area, each cell is normally physically positioned so that its area of coverage is adjacent to and overlaps the areas of coverage of a number of other cells. When a mobile unit moves from an area covered by one base station to an area covered by another base station, communications with the mobile unit are transferred (handed off) from one base station to another in an area where the coverage from the adjoining cells overlaps. Because of this overlapping coverage, the channels allotted to the individual cells are carefully selected so that adjoining cells do not transmit or receive on the same channels. This separation is typically accomplished by assigning a group of widely separated non-interfering channels to some central cell and then assigning other groups of widely separated non-interfering channels to the cells surrounding that central cell using a pattern which does not reuse the same channels for the cells surrounding the central cell. The pattern of channel assignments continues similarly with the other cells adjoining the first group of cells.
When collecting data for analysis and optimization of a wireless system, the present invention utilizes measured path lost data as the foundation for analyzing cellular systems. Path lost is defined as the attenuation in a source antenna (sector) and a point on the terrain of the wireless system. The path lost is measured by subtracting the RSSI of a signal at a location on the terrain from a known transmitted signal level for the received signal: Path Loss [dB]=Ptx [dBm]−RSSI [dBm]
In order to obtain RSSI values at various locations, a vehicle equipped with an RSSI scanning receiver and GPS receiver is driven by a technician on a predetermined route through the wireless system. During the drive, data is collected from the RSSI scanning receiver coordinated with the GPS receiver and log on a computer in the vehicle. The long information includes (at a minimum) GPS position (latitude, longitude), RSSI value and receive channel information.
Once data has been collected, it is post processed. The post processing involves associating each measurement to sector in calculating the path loss. The association of includes matching channel (and color code) from measurements to a channel (and color code) used on a sector. The path loss calculation requires knowledge of least one reference channel used on a sector and its transmit power.
To analyze a cellular system, it is necessary to have path loss information each location for sectors that may serve and sectors that may interfere at a particular location. For example, if a system requires a S/I (signal to interference ratio) of 20 dB for interference free communication, the collection must obtain measurements for all sectors that appear 20 dB below the serving signal level. With this information, it is possible to automate optimization tasks, such as determining frequency assignments that provide a desired quality of service.
Other cellular analysis tools rely on propagation modeling techniques to estimate the path loss between sectors and the locations on the terrain. Due to the effects of terrain, foliage, and man-made obstructions, it is impossible to estimate path loss was sufficient confidence to properly optimize a wireless system.
Since channels are reused in a wireless system, the challenge in obtaining measured path lost data involves measuring a signal from a sector independent of the signals transmitted by other sectors using the same channel. The following describes the current methodology and a proposed methodology for collecting measured path lost data.
Overview of Cellular Technologies.
IS 136 is a TDMA technology used extensively in North and South America. Each channel, as illustrated in FIG. 1, is 30 kHz and is comprised of six time slots per frame. The duration of a time slot is 6.67 milliseconds, producing a frame duration of 40 milliseconds. Most cellular operators use IS 136 in full duplex mode, such that each control/voice path requires to time slots per frame. Therefore each channel may serve up to three users.
On average, a typical sector, as illustrated in FIG. 2, contains six channels. One channel is designated as a control channel. The control information occupies two time slots per frame. The control channel transmits continuously at a constant power level. The remaining time slots provide two voice paths in the control channel.
In order to minimize interference in a wireless system, a voice channel becomes inactive when there are no time slots in use. Most systems employ a method for packing voice paths into channels that are in use in order to minimize the number of active channels in a system. Often, a priority is given to the control channel since it will always be in use and assigning voice paths into the control channel does not activate a new channel in the cellular system.
The Digital Verification Color Code DVCC room is transmitted on each time slot used for a voice path. The DVCC uses an 8 bit code with four parity bits, allowing 255 color code values (value 00 H. is not available for use close friends. Typically, each channel at a cells site uses the same DVCC assigned by the operator.
When a voice path is not used on a voice or control channel, the system assigns the DVCC value 255 to the on occupied time slots. Therefore, the channel/color code combination cannot be used as a unique identifier to determine the source sector for the signal. Unoccupied time slots from a number of cell sites and/or sectors will contain the same DVCC value.
In addition, the DVCC is not included in time slots used for the control path. Therefore the system must have voice traffic in order for the detectors to obtain the DVCC for a transmission. It is possible to obtain the DVCC from time slots used for the control path, but this would require decoding multiple control time slots.
The use of downlink power control becomes an issue for any measurements from voice channels. Power control dynamically adjusts the channel power level to maintain adequate level of service for the user while minimizing interference in the system. Since the transmit power fluctuates, it is not possible to use these channels for obtaining path lost measurements. In this situation, it is still possible to obtain measurements and color code from the voice time slots on the control channel.
iDEN is a proprietary cellular technology developed by Motorola. One of the obvious challenges in analyzing a iDEN signal is that the signal format specifications are not readily available. Each channel in and iDEN system occupies a bandwidth of 25 kHz. There are typically 426 base station radios and each sector. Unlike IS 136, radios in and iDEN system transmit constantly. Since the interference is constant, it is important to have a method for decoding the color code even in the presence of significant interference.
GSM, the standard sailor protocol deployed in Europe, is essentially the same as DCS 1800 and PCS 1900 (used in the United States). Many cellular operators are progressing toward a technology called EDGE that offers enhanced capabilities. EDGE is based in the same signal protocols has GSM.
The GSM channel is 200 kHz and utilizes 8 time slots. At least one channel on each base station contains time slots used for control information. Part of this control information includes the BSIC (base station identity code), which has 64 possible values. GSM channels used for voice traffic may only use power control and frequency hopping. Power control is used to minimize interference while frequency hopping tends to randomize the interference. This functionality is not allowed on the control channel.
IS95 (CDMA) unlike other cellular technologies that rely on frequency separation to minimize interference, uses the spread spectrum to combat interference. This allows the same channels to be used at every sector. IS95 uses multiple levels of encoding to reduce interference. The Welsh code is used within a channel on a sector to provide control information with voice paths. The pseudo-random noise (PN) code is used to identify each sector within a cellular system. There are 64 Walsh codes on each channel. The pilot channel uses Walsh code 0 and is transmitted continuously at a constant power level. Walsh codes used for voice paths use power control. The PN code is a sequence of 32,768 chips. Each sector transmits the same PN code at a different offset in time. The code is divided into 512 offset positions (PN offsets). A system will usually use a subset of these offsets, since signals propagate in a large distance may be delayed such that they appear as an adjacent offset.
Each base station is synchronized using a GPS timing reference. This allows transmission of the PN sequence at the proper offset position. By using this GPS timing reference to synchronize the receiver equipment to the system, it is possible to approximate propagation distance from a sector to the measurement location.
While traveling through a wireless system, a large number of signals from many different cells and sectors are received at each point throughout the system. The mobile units within the system receive signals from antennas directed towards the mobile units location, as well as from the back and/or side lobes of antennas directed away from the physical location of the mobile unit. The receipt of signals from antennas not directed towards the mobile unit is often referred to as back scatter or side scatter. Often adjacent or overlapping cells will transmit on the same frequency and both will be received by a mobile wireless unit. However, because of the digital codes identifying each channel, the mobile wireless unit can process the appropriate signal and ignore any additional reception.
It is desirable to provide a process for modeling a wireless system to determine the propagation of RF signals within the wireless system and for determining the effect on the system performance of proposed antenna changes, such as relocation, redirection, or substitution of antennas with different characteristics.